This invention relates generally to rotation sensors and particularly to ring laser gyroscope rotation sensors. Still more particularly, this invention relates to apparatus and methods for reducing errors in measurements made with ring laser gyroscopes caused by the tendency of the counterpropagating beams of ring laser gyroscopes to lock to a common frequency at low rotation rates.
A ring laser gyroscope employs the Sagnac effect to detect rotation. Two counter propagating light beams in a closed loop will have transit times that differ in direct proportion to the rotation rate of the loop about an axis perpendicular to the plane of the loop. The planar ring laser gyroscope has the simplest type of optical path. However, other path geometries provide advantages over the planar path.
There are in general two basic techniques for utilizing the Sagnac effect to detect rotations. A first technique is the interferometric approach, which involves measuring the differential phase shift between two counterpropagating beams injected from an external source, typically a laser, into a Sagnac ring. The ring may be defined by mirrors or by a coil of optical fiber. Beams exiting the path interfere and create a pattern of light and dark lines that is usually called a fringe pattern. Absolute changes in the fringe pattern are indicative of rotation of the ring. The primary difficulty with such devices is that the changes are very small for rotation rates of interest in guidance applications.
The ring laser gyroscope uses the second technique for utilizing the Sagnac effect to detect rotations. In a ring laser, resonant properties of a closed cavity convert the Sagnac phase difference between the counter propagating beams into a frequency difference, which is more easily measured than the absolute phase shift. Ring laser gyroscopes may be classified as passive or active, depending upon whether the gain medium is external or internal to the cavity. In the active ring laser gyroscope the cavity defined by the closed optical path becomes an oscillator. Output beams from the two directions interfere to give a beat frequency that is a measure of the rotation rate. The oscillator approach has the advantage that the frequency filtering properties of the cavity resonator are narrowed by many orders of magnitude below the passive cavity to give the potential for simple configurations for very precise rotation sensing. To date, the major ring laser gyroscope rotation sensor effort has been put into the active ring laser. Presently all commercially available optical rotation sensors are active ring laser gyroscopes.
A ring laser gyroscope has a sensor axis that passes through the closed path traversed by the counterpropagating beams. For a planar path, the sensor axis is conveniently normal to the path. In an out of plane gyro, the sensor axis may be a line normal to the projection of the path upon a plane. When the ring laser gyroscope is not rotating about its sensor axis, the optical paths for the two counterpropagating beams have identical lengths so that the two beams have identical frequencies. Rotation of the ring laser gyroscope about its sensor axis causes the effective path length for light traveling in the direction of rotation to increase while the effective path length for the wave traveling opposite in direction to the rotation decreases.
When the rotation rate of the ring laser gyroscope is within a certain range, the frequency difference between the beams disappears. This phenomenon is called frequency lock-in, or mode locking, and is a major difficulty with the ring laser gyroscope because at low rotation rates the ring laser gyroscope produces a false indication that the device is not rotating. If the rotation rate of a ring laser gyroscope starts at a value above that where lock-in occurs and is then decreased, the frequency difference between the beams disappears at a certain input rotation rate. This input rotation rate is called the lock-in threshold. The range of rotation rates over which lock-in occurs is generally called the deadband of the ring laser gyroscope.
Lock-in arises from coupling of light between the beams. The coupling results primarily from backscatter off the mirrors that confine the beams to the closed path. Backscatter causes the beam in each direction to include a small component having the frequency of the beam propagating in the other direction. The lock-in effect in a ring laser gyroscope is similar to the coupling that has been long been observed and understood in conventional electronic oscillators.
In addition to causing erroneous rotation rate information to be output from a ring laser gyroscope, lock-in causes standing waves to appear on the mirror surfaces. These standing waves may create a grating of high and low absorption regions, which creates localized losses that increase the coupling and the lock-in. The mirrors may be permanently damaged by leaving a ring laser gyroscope operating in a lock-in condition.
Any inability to accurately measure low rotation rates reduces the effectiveness of a ring laser gyroscope in navigational systems. Therefore it is well known that a ring laser gyroscope requires means for circumventing mode locking.
There are several known attempts to solve the problems of lock-in. Currently the primary method for reducing the effects of mode locking involves mechanically oscillating the ring laser gyroscope about its sensor axis so that the device is constantly sweeping through the deadband and is never locked therein. This mechanical oscillation of the ring laser gyroscope is usually called dithering. A typical ring laser gyroscope may be dithered at about 400 Hz with an angular displacement of a few arc minutes. Although this method has proven effective in some guidance applications, there are disadvantages associated with mechanical dithering. Dither suspension and drive mechanisms are mechanically complex, and fail to completely eliminate the effects of residual backscatter coupling. Some guidance applications cannot tolerate the amount of mechanical vibration required to mechanically dither the ring laser gyroscope frame.
Mirror dither is another approach that has been investigated in attempts to reduce the effects of lock-in. One or more of the mirrors that define the optical path may be oscillated at a very small amplitude. The Doppler effect causes a difference between the frequency of backscattered light and forward reflected light. Transverse dithering of all four mirrors of a rectangular gyro shifts only the frequency of the backscattered beam. However, transverse mirror dither is difficult to implement because of the large amount of energy required to move mirrors that are mounted to the gyro body. Longitudinal mirror dither is easier to implement, but it shifts the frequencies of both the forward and backscattered light. Therefore, the analysis of signals in a longitudinally mirror dithered gyro is complicated.
Multioscillator ring laser gyroscopes have also been proposed to overcome the effects of mode locking. The term "multioscillator" refers to the four modes of electromagnetic energy that propagate simultaneously in the cavity as opposed to the usual pair counterpropagating linearly polarized modes that exist in the conventional two mode gyro. Multioscillator ring laser gyroscopes are described in U.S. Pat. No. 3,862,803 to Yntema, Warner and Grant and U.S. Pat. No. 3,741,657 to Andringa.
The basic multioscillator ring laser gyroscope operates with left circularly polararized (LCP) and right circularly polarized (RCP) light beams and uses a Faraday effect glass device within the cavity or a magnetic field on the gain plasma to provide a phase shift between the counterpropagating waves to prevent mode locking. Reflections from the intracavity element and instabilities in the magnetic field cause difficulties that have yet to be overcome in attempts to build a navigation grade multioscillator ring laser gyroscope.
In order for the multioscillator ring laser gyroscope configuration to operate, the cavity must include means for separating the operating frequencies of the LCP and the RCP gyroscopes. This separation is the reciprocal bias, which is in addition to the non-reciprocal bias that operates between modes propagating in opposite directions. Early multioscillator ring laser gyroscopes used an intracavity optically active crystalline element to provide the reciprocal splitting. Later Dorschner et al. disclosed in U.S. Pat. No. 4,482,249 an out-of-plane light path that provided the required reciprocal splitting without using an intracavity element. The amount of reciprocal splitting is easily controlled by changing the amount of nonplanarity. However, the multioscillator ring laser gyroscope disclosed by Dorschner et al. still required an intracavity element to create the nonreciprocal biasing.
Intracavity elements are undesirable for many reasons. Apart from causing additional complexity in the manufacture and assembly, intracavity elements contribute to cavity losses and to backscatter. The cavity losses increase random walk, and the backscatter causes scale factor errors. Intracavity elements also are a source of thermal drift effects, etaloning within the cavity and long term drifts since element stress and position within the cavity change with age. There are also concerns regarding the effects of nuclear radiation on such elements.
Both Dorschner in U.S. Pat. No. 4,229,106 and Sanders in U.S. Pat. No. 4,231,705 have suggested the removal of the intracavity element by using the Verdet constant of the gas gain plasma itself as a source for a non-reciprocal bias. The anomalous dispersion of the gain medium at the lasing frequency and the additional length of the plasma region over any possible intracavity element can mean that comparable bias shifts are obtainable. However, the Verdet constant is in effect proportional to the overall cavity loss and can be considerably reduced for low-loss cavities. This configuration involves the application of an axial magnetic field of typically about 100 gauss to the gain plasma and is sometimes referred to in the literature as a ZEELAG device (ZEEman LAser Gyro). This ZEELAG configuration proved to be unsuccessful as an accurate laser gyroscope because of large sensitivities of the bias to both changes in the cavity length (detuning) and in temperature. The primary origin of these effects is directly related to cavity detuning. Because the LCP and RCP gyros must operate at greatly different points on their respective dispersion curves, there is a large differential change in their Verdet constants as the cavity detuning is changed. Therefore, the nonreciprocal bias magnitude for the two gyros do not track each other closely enough to provide high accuracy operation as a rotation sensor. Temperature variations also cause differential changes in the shape of the dispersion curves and produce errors.
Since the shortcomings of the ZEELAG configuration have been demonstrated, efforts on the multioscillator ring laser gyroscope have avoided the use of large axial magnetic fields on the gain plasma and have concentrated on minimizing the error created by the intracavity glass element positioned in a large (.about.kilogauss) axial field such as disclosed in U.S. Pat. No. 4,548,501 by Dorschner et al.
Small axial fields of less than one gauss have been suggested for use on the gain plasma with the purpose of compensating for the differential dispersion created by the non-reciprocal bias. Such methods are described in U.S. Pat. No. 3,973,385 to Ferrar and in U.S. Pat. No. 4,470,701 to Smith.
U.S. Pat. No. 928,069 by the present inventor discloses a ring laser gyroscope in which an axial magnetic field is applied to the gain region in order to suppress two of the four modes that would normally lase in a nonplanar four-mirrored clear path cavity with circularly polarized modes. The disclosure of that application is hereby incorporated by reference into the present disclosure. This arrangement creates a configuration that relies on the reciprocal splitting produced by a nonplanar cavity to provide a bias between the two remaining modes in the cavity. This two mode device has the drawback of depending on the stability of the reciprocal splitting produced by the nonplanar geometry. This frequency difference is subject to changes in the cavity mirror phase retardation, which is induced by temperature changes and plasma damage effects. Geometry changes such as those induced by mirror tilt when the diaphragm mirrors are moved, as described in U.S. Pat. No. 4,383,763 to Hutchings to provide cavity length control, also affect the frequency difference.